Thin film cryotron



Feb. 8, 1966 G. A. ALPHONSE 3,234,439

THIN FILM CRYOTRON Filed May 1, 1962 2 Sheets-Sheet l United States Patent O 3,234,439 IIHN FILM CRYOTRUN Gerard A. Alphonse, New York, NX., assigner to Radio Corporation of America, a corporation of Delaware Filed May 1, 1962, Ser. No. 191,605 8 Claims. (Cl. 317-434) This invention relates to cryoelectric switching elements and, more particularly, to improved thin lm cryotrons.

An object of the invention is to provide a thin lilm cryotron which operates etiiciently with a relativelyY wide control electrode. t

Another object of the invention is to provide a thin lm cryotron which is capable of `operating at relatively highy gain even though the control electrode width is relatively large. v

The cryotron of the invention includes a gate electrode, and arcontrol electrode coupled to the gate electrode. The two electrodes are locatedover a groundplane which is formed with an aperture located beneath the area at which the gate and control electrodes are coupled to one another.`

The invention is described in greater detail below and is illustrated in .the following drawing of which:

FIG. 1 is a perspective, partially cut away view of a prior Iart cryotron;

FIGS. 2a and 2b are drawings to illustrate the operation of the prior art cryotron of FIG. l; FIG. 3 is a perspective, partially cut away view of a cryotron according to the present invention;

FIGS. 4a-4c are drawings to illustrate the operation of the cryotron of the present invention;

FIGS. 5a and 5b` are sketches which also illustrate the operation of the cryotron of the present invention; and

FIG. 6 shows an embodiment of the invention in which one control electrode is coupled to two gate electrodes.

In the discussion which follows, it is assumed that the cryotrons are in a low temperature environment at which superconductivity is possible.

The prior art cryotron shown in FIG. 1 includes a ground plane 10 which may be made of lead, for example, over a substrate 11, for example, of glass, covered with a thin layer of insulation 12 which maybe silicon monoxide. A gate electrodeld which may be formed of tin isdeposited on the insulation layer. A control electrode 16 which` may be made of lead iscoupled to the gate electrode. The control electrode extends at approximately 90 to the gate electrode and is insulated therefrom by thin insulation layer 18.`

In the operation of thecryotron, the gate and control electrodes may initially both `be in the superconducting condition. A current pulse applied to the control electrode 16 causes-the area of the gate electrode 14 lying beneath the control electrode to `become resistive. The leadground plane `10 is important inthe operation of the cryotron for a numberofreasons. For one thing, it re- `duces the inductance of` the gate and control electrodes and in this way. increases the operating speed `of `the cryotron. This is discussedin an article'by A. E. Slade,

.Cryotron Characteristics and Circuit Applications, Pro ceedings ofthe IRE, September 1960, page 1569, and a second article in the same issue by C. R. Smallman, Thin Film Cryotrons, at page 1562. VA further advantage of employing the ground plane is that it makes the `current distribution uniform in the tin and lead electrodes and in this way improves the reproducibility of performance characteristics obtainable with the cryotron. The ground plane also causes the distribution of the magnetic field produced by the control electrode to be uniform. The uniform magnetic eld distribution is shown in FIG. 2a and the uniform current distribution is shown in FIG. 2b.

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To obtain any appreciable gain from the cryotron shown in FIG. 1, the control electrode width should be made as small as possible with respect to the width of the gate electrode. In general, g, the gain of the cryotron equals where k is a constant describing the eihciency of the cryotron, wg is the gate electrode width and wc is the control electrode width. Making the control electrode very narrow introduces some fabrication problems and increases the inductance of the control circuit. Even more important, the control electrode wc cannot be made too small as this would affect its current carrying capacity. In other words, if the widthfand therefore the cross-sectional area of the control electrode were made too small, the control electrode would switch from its superconductive to its normal resistive condition prior to the time that a magnetic field of suicient intensity to switch the gate electrode normal could be generated by the control electrode current.

The cryotron of the present invention is illustrated in FIG. 3. There are two important diiierences between lthiscryotron and the one of FIG. 1. The rst is in the width of the control electrode. Rather than making the `control electrode narrow, it is made relatively wideas wide as or even wider than the gate electrode. The second difference is that the ground plane is not continuous. Instead, an aperture or slot is formed in the ground plane immediately beneath the place where the control electrode crosses the gate electrode. The silicon monoxide insulator layer over the ground plane is continuous, however, a portion is broken away in FIG. 3 to show the aperture in the ground plane. Elements of the structure of FIG. 3 which are similar to the corresponding elements of FIG. 1 are legended with the same reference numerals primed. FIG. 5a is a plan view of the cryotron of FIG. 3. The insulation between the gate and control electrodes is shown in FIG. 3 but not shown in FIG. 5a. The location of the aperture in the ground plane beneath the control and gate electrodes is shown by dashed lines in FIG. 5a.

In the absence of the ground plaire, that is, over the ground plane aperture, the distribution of current carriers in the control electrode is not uniform. Instead, the current carriers concentrate at the opposite edges of the control electrode I6 as is shown in FIG. 4c. The concentration of carriers causes a concentration of magnetic eld at the opposite edges of the control electrode 16 as is shown in FIG. 4c. The concentration of carriers causes a concentration of magnetic field at the opposite edges of the control electrode. The magnetic eld distribution normal and tangential to the surface I9 of the control electrode, as a function of the distance from the center '(the longitudinal axis) of the control electrode 16', is shown in FIGS. 4a and 4b, respectively. The width of the control electrode is w, and iw/Z in FIGS. 4a and 4b represent the respective outer lateral edges of the control electrode. FIG. 4b indicates that there is a strong normal component of the magnetic eld. FIGS. 4a and 4b indicate that the magnetic field drops oi rapidly as one moves away from the outer edges of the control electrode in the tangential direction.

In the operation of the cryotron of FIG. 3, when a control current of suilicient magnitude is appli-ed to the control electrode,`the areas of the gate electrode 14 adjacent to the opposite lateral edges of the control electrode 16', that is, the areas of the gate electrode receiving the highest concentration of magnetic field become normal. This does not appreciably slow down the switching of the cryotron.

An important advantage of the cryotron of FIG. 3 is that the control electrode 16 is relatively Wide and can be easily deposited through relatively large masks. Thus, the fabrication problem is relatively simple. Further, the current carrying capacity of the control electrode 16 is quite large. Nevertheless, as the current concentrates at the edges of the control electrode 16', the effective width of the control electrode is relatively small. The absence of the ground plane beneath the cross-over area does not seriously adversely aect the inductance of the control electrode. As pointed out on page 1570 of the periodical above, in the absence of the ground plane, the inductance of control electrode increases. However, the inductance of the control electrode is also inversely proportional to its width. Therefore, the increase in width of the control electrode 16 tends to decrease its inductance and thereby tends to compensate for the increased inductance due to the absence of the ground plane. lFurther, the time constant of the cryotron is proportional to I, the inductance of the control electrode, divided by r, the resistance of the gate electrode. As the control electrode is much wider in the embodiment of FIG. 3y than in FIG. l, it makes a much larger area of the gate electrode normal as indicated in FIG. 5b. Therefore, r in the formula discussed above increases and the time constant tends to decrease.

In summary, the cryotron of the invention is relatively easy to fabricate and is mechanically strong. The control electrode has high current carrying capacity and, in view of the concentration of current carriers at its edges, produces an intense magnetic field. Further, this eld can easily be increased by increasing the current applied to the control electrode without danger of making the control electrode normal due to the self-quenching action of the control current. The gain of the cryotron of the invention is high. Its speed is also high as it has relatively low inductance and relatively large resistance. Finally, the cryotron can be made of relatively small size just as the prior art cryotron.

The embodiment of the inventiony shown in FIG. 6 includes twov gate electrodes 22 and 24 and a single control electrode 26. The insulation between the control electrode and gate electrodes is present but not shown in FIG. 6. The insulation between the ground plane and the gate and control electrodes is also present. There is an aperture 28 in the ground plane located beneath one edge 30 of the control electrode 26 of the cryotron 22, 26. There is a second aperture 32 located between the edge 34 of control electrode 2,6 and beneath the gate electrode 24. In each case, the aperture does not extend beneath both edges of the control electrode. However, in each case, there is a buildup of carriers and a concentration of magnetic field at the edge of the control electrode located over the aperture. This permits the control electrode to switch the gatev electrode normal at relatively low values of current applied to the control electrode 26.

The thicknesses of superconducting and insulating layers in the embodiments of the invention discussed above may be similar to those discussed in the periodical reference cited. The widths of the control and gate electrodes depend upon the desired circuit speed, the desired current carrying capacity, the number of inputs and outputs (fanin, fan-out), the size of the substrate and so on. The minimum widthy is mainly determined by fabrication technology, required line width toleran, maximum' permissible inductance, and current required. At the present state of the art, the gate electrode Width may be as small as 0.005. The maximum line width is determined by packingA density requirements. Typical widths may vary from 0.01 to 0.l. In each case, the controlline- Width at the place where the control and gate electrodes cross may be equal to or greater than the gate electrode width.

As can be seen in FIGS. 4a and b, both the tangential and normal components of the magnetic field decay rapidly beyond the ylateral edges of the control electrode. If W is the control electrode width, the magnetic eld drops to about lo of its maximum value at a distance as small as 0.05W away from the edge (see in this connection Myer, N. H., An Analog Solution for the Static London Equations of Superconductivity, Proceedings of the IRE, 48, September 1960, pages 1603-1607). Accordingly, the minimum dimension of the aperture in the ground plane for an embodiment of the invention such as shown in FIG. 3 is about 1.1 times the corresponding dimensions of the control and gate electrodes where they cross. For example, if the gate electrode is 0.02 wide and the control electrode 0.02" wide, then the aperture may be 0.022 x 0.022. If the gate electrode is 0.02" wide and the control electrode is 0.03" wide, then the aperture may be 0.022 x 0.033 and so on. The aperture may be square, rectangular, circular or of other shape. However, it should not be made much bigger than necessary as this would add unnecessarily tothe inductance of the circuit.

What is claimed is:

1. A thin tilm cryotron comprising, in combination, a gate electrode; a control electrode coupled to the gate electrode; and s-olely a single ground plane on which said two electrodes are supported, said plane being formed with an aperture located beneath the area at which the two electrodes are coupled.

2. A thin film cryotron comprising, in combination, a thin film gate electrode; a thin film control electrode of roughly the same width as the gate electrode insulated from and electrically coupled to the gate electrode; and an insulated ground plane which provides the sole magnetic lield shielding elTect for said cryotron on which said two electrodes are supported, said plane being formed with an aperture located beneath the area at which the two electrodes are coupled.

3. A cryotron comprising, in combination, a thin lm gate electrode; a thin iilm control electrode having a Width comparable to that of the gate electrode, insulated from and electrically coupled to the gate electrode, said two electrodes extending at substantially toy one another; and an insulated ground planewhich pro-Vides the sole magnetic field shielding effect for said cryotron on which said two electrodes are supported, said plane being formed with an aperture of larger size than the cross-over area of said two electrodes located beneath the area at which the two electrodesV are coupled.

4. A thin film cryotron comprising solely a single superconducting ground plane formed with an aperture; an insulating layer over the ground plane; a superconducting gate electrode on the insulating la-yer extending over said aperture, said gate electrode having a width somewhat smaller than that of the aperture; and a control electrode insulated from and passing over the gate electrode at an angle thereto at least one edge of said control` electrode passing over said aperture. l

5. A thin film cryotron comprising solely a single superconducting ground plane formed with an aperture; an insulating layer over the ground plane and aperture; a superconducting gate electrode on the insulating layer extending over said aperture, said gate electrode having a width somewhat smaller than that of the aperture;` and a control electrode of a Width comparable to that of the gate electrode insulated from and passing. over the gate electrode at an angle thereto, at least one edge of said control electrode passing over said aperture.

6. A thin lm cryotron comprising solelya single superconducting ground plane formed with an aperture; an insulating layer over the ground plane;y a superconducting gateelectrode on the insulating layer extendingover said aperture, said gate electrode having: a width somewhat smaller'than that of the aperture; and a control electrode of a width comparable to that of the gate electrode insulated from and passing over the gate electrode at substantially a right angle thereto, both edges of said cona thin-hlm con-trol electrode formed of superconductor trol electrode passing over said aperture. material having a substantial width comparable to 7. A thin-lrn cryotron comprising: that of the gate electrode and crossing the gate eleca thin-lm gate electrode formed of a superconductor trode at substantially right angles at a region thereof material; 5 where both the gate and control electrodes are suba thin-film control electrode formed of superconductor stantially unshielded, WheIeblI When the Current iS aP- material having a substantial width, comparable to plied to the control electrode, the magnetic field therethat of the gate electrode, and electrically coupled by produced as a component of substantial magnitude to the gate electrode at a region thereof Where both WhiCh iS Dermal t0 the Surface 0f the gate eleelede;

the gate and control electrodes are substantially unand shielded, whereby when a current is applied to the ground plane means formed of a superconductor matecontrol electrode, the magnetic eld thereby produced Tiel 011 Which the CTI/OUCH S SUPPOTed, Said ground has a component of substantial magnitude which is Pl'clhe means being formed With ah apeffhfe beneath normal to the Surface of the gate electrode; and the region at which the gate and control electrodes ground plane means formed of a superconductor mate- CFOSS, whereby Sad gate and Control eleerOdeS rerial on which the cryotron is supported, said ground mam Substahtlany uhshielded at the region at Which plane means being formed with an aperture beneath heytlclross, but are shlelded for the remainder of their eng the area at which the gate and control electrodes are electrically coupled, whereby said gate and control electrodes remain substantially unshielded at the Referens Cited by the Examiner area at which they are coupled, but are shielded for UNITED STATES PATENTS the remainder of their length. 3,086,130 4/ 1963 Meyers et al. 307--88-5 8. A thin-hlm cryotron comprising: a thin-film gate electrode formed of superconductor JOHN W' HUCKERT Pmary Examine"- Inaterial; GEORGE N. WESTBY, Examiner. 

1. A THIN FILM CRYOTRON COMPRISING, IN COMBINATION, A GATE ELECTRODE; A CONTROL ELECTRODE COUPLED TO THE GATE ELECTRODE; AND SOLELY A SINGLE GROUND PLANE ON WHICH SAID TWO ELECTRODES ARE SUPPORTED, SAID PLANE BEING FORMED WITH AN APERTURE LOCATED BENEATH THE AREA AT WHICH THE TWO ELECTRODES ARE COUPLED. 